Cisplatin (cDDP or cis-diamminedichloroplatinum(II), FIG. 1) is the most widely used of the platinium chemotherapeutic compounds approved for use in humans and is currently indicated for the treatment of testicular, ovarian, and head and neck tumors and, in combination with other agents, for the treatment of squamous cell and small cell lung carcinomas. The anti-tumor activity of cisplatin is believed to result from the loss of the chlorine ligand(s) in vivo to form reactive mono- or di-aqua complexes, which, in turn, form intra- and inter-strand DNA cross-links in tumor cells, leading to cell death.
There are, however, significant limitations to the use of cisplatin due to its nephrotoxicity and ototoxicity. Many novel small molecule Pt complexes have been made and tested in hope of finding new compounds having improved therapeutic indices (the ratio of the maximum tolerated dose to the minimum effective dose). For example, at the Institute for Cancer Research in the U.K., it was demonstrated that replacing the chlorine atoms with other leaving groups could give compounds exhibiting lower nephrotoxicity. This led to the discovery of carboplatin (FIG. 1), a cisplatin analog in which the chloride ligands are replaced by a 1,1-cyclobutane-dicarboxylic acid chelate. The chelate is less labile than the chlorides of cisplatin so higher doses of carboplatin are required to achieve a tumorcidal effect similar to that of cisplatin. Carboplatin's higher therapeutic index and different toxicity profile, however, negates this potential disadvantage. The dose-limiting toxicity of carboplatin is myelosuppression.
Oxaliplatin (FIG. 1) is another Pt chelate approved for human use. Oxaliplatin was the result of research into the effect of replacing both the non-labile (ammine, ammonia) and the labile (chloride) ligands of cisplatin with other groups. In oxaliplatin, the ammonia ligands are replaced with a trans-1R,2R-diaminocyclohexane (1R,2R-DACH) chelate and the chloride ligands are replaced with an oxalic acid chelate. Oxaliplatin is indicated in the treatment of colorectal cancer. The dose-limiting toxicity of oxaliplatin is sensory neuropathy.
Many other small molecule Pt complexes have been made and tested but so far only slight improvements in efficacy and therapeutic index have been achieved. A number of attempts to improve the therapeutic index of the approved platinum complexes have involved either combination therapy, for example, the co-administration of cisplatin and paclitaxel, or formulation changes such as delivery in liposomes.
Another approach to improving the therapeutic index of Pt complexes would be to target the complexes to tumor cells. Conventional small molecule Pt complexes such as cisplatin, carboplatin, and oxaliplatin are not specifically targeted to tumor cells and, following intravenous administration, they diffuse into normal cells as readily as into tumor cells. One method of tumor targeting that has been extensively studied with regard to non-Pt chemotherapeutics involves the attachment of the chemotherapeutic compound to a polymer or other macromolecular structure such as a dendrimer, a serum protein or an antibody. It has been demonstrated that the concentration of polymers and nanoparticles in tumors exceeds their concentration in normal tissue following intravenous administration. The mechanism for this preferred tumor accumulation has been termed the “enhanced permeability and retention” (EPR) effect. Essentially, tumor endothelial cells layers tend to be more ‘leaky’ than normal endothelial cell layers so that large chemical entities such as polymers and nanoparticles more readily cross the endothelial cell layer of the tumor vasculature and enter the interstitial areas of the tumor (“enhanced permeability”). Furthermore, lymphatic drainage of extracellular fluid in tumors is much less efficient than that of normal cells, thus reducing the rate of efflux of polymers and nanoparticles from tumors compared to normal tissue (“enhanced retention”).
Examples of constructs that provide passive targeting of chemo-therapeutic agents to tumors through the EPR effect include doxorubicin attached to a linear polyhydroxypropylmethacrylamide polymer (poly(HPMA)) through a tetrapeptide designed to be cleaved by lysosomal enzymes. This water-soluble conjugate, termed “PKI,” has been the subject of numerous publications describing its chemistry, pre-clinical testing and clinical evaluation. Similarly, poly(HPMA) has been conjugated with paclitaxel and camptothecin for selective delivery of these chemotherapeutic molecules to tumors.
In addition to passive tumor targeting, it may also be possible to target Pt complexes to tumors using active mechanisms such as coupling of the Pt complex to a moiety that binds to a receptor which is up-regulated in tumor cells compared to normal cells. A wide variety of such up-regulated receptors are known (Heppeler, et al., 2000; Schlaeppi, et al., 1999; Sudimack, et al., 2000; Dubowchik, et al., 1999; Weiner, 1999; Buolamwini, 1999). Examples of receptor binding agents include monoclonal antibodies, peptides, somatostatin analogs, folic acid derivatives, lectins, vitamins, such as cobalamin and its derivatives, biotin and polyanionic polysaccharides. Studies of Pt conjugated with monoclonal antibodies (McIntosh, et al., 1997; Hata, et al., 1992), steroids (Gust, et al., 1995; DiZio, et al., 1992, Gibson, et al., 1990) and folic acid (Vitols, et al., 1987) have been reported but none have been evaluated in the clinic.
It is also possible to combine passive and active targeting. This is exemplified by PK2, a compound comprising poly(HPMA) to which doxorubicin is attached through an enzyme cleavable peptide and to which galactose, a carbohydrate with strong affinity for the asialoglycoprotein receptor, which is highly concentrated in the liver, is also conjugated.
What is needed is a means of preparing a pharmaceutically acceptable Pt-polymer complex, i.e., a complex of known and reproducible structure and purity that can be used to target tumors using active and passive targeting technologies. The present invention provides such a means and the Pt complexes prepared thereby.